The present invention relates to a hub unit for supporting a wheel shaft, which is utilized for supporting a wheel of a vehicle rotatively relative to a suspension system.
Wheels of a vehicle are supported on the suspension system by means of hub units for supporting wheels. FIG. 8 shows one example of a hub unit for supporting a wheel described in U.S. Pat. No. 5,490,732. A hub unit 1 for supporting a wheel comprises a shaft member 2, a pair of inner rings 3a, 3b, an outer ring 4, and a plurality of rolling elements 5, 5. A flange (a first flange) 6 for supporting a wheel is formed along the outer peripheral surface of outer-edge portion of the shaft member 2 (here the expression "out or outside" designates a direction toward the outside of the vehicle in a widthwise direction when the hub unit is mounted on the vehicle, that is, a leftward direction in FIG. 8, and in contrast, an expression "in or inside" designates a direction toward the center of the vehicle in the widthwise direction, that is, a rightward direction in FIG. 8.). A stepped portion 7 is formed along the base of the flange 6 in the vicinity of the axial center of the axial member 2.
The pair of inner rings 3a, 3b are provided and fitted around the outer surface of the axial member 2 which ranges from the middle portion to the inner end of the axial member. The outer side surface of the inner ring 3a abuts a vertical surface of the stepped portion 7, and the outer side surface of the inner ring 3b abuts the inner side surface of the inner ring 3a. A cylindrical portion 8 is formed in the inner end surface of the shaft member 2. A caulked portion 9 is formed by bending, in a diametrical outward direction, the edge of the cylindrical portion 8 which protrudes toward the inside beyond the inner end surface of the inner ring 3b. The pair of inner rings 3a, 3b are sandwiched between the vertical surface of the stepped portion 7 and the caulked portion 9.
The plurality of rolling elements 5, 5 are provided between a pair of outer (first and second) raceways 10, 10 formed along the inner peripheral surface of the outer ring 4 and a pair of inner (first and second) raceways 11, 11 formed respectively along the outer peripheral surfaces of the inner rings 3a, 3b. Although balls are used as the rolling elements 5, 5 in an illustrated example, tapered rollers may also be used as the rolling elements for a heavy hub unit for supporting a vehicle wheel. Alternatively, the (first) inner raceway formed adjacent to the flange 6 may be directly formed along the outer peripheral surface of the shaft member 2, and the outer inner ring 3a may be omitted in some cases. In such a case, the stepped portion 7 is formed in an inner position relative to the inner ring 3a shown in FIG. 8.
The foregoing hub unit 1 is locked to the vehicle by securing the outer wheel 4 to the suspension system through use of an outwardly extending flanged mount (a second flange) 12 formed along the outer peripheral surface of the outer wheel 4, and by fastening the wheel to the flange 6. As a result, the wheel can be rotatively secured to the suspension system.
In the case of the conventional construction shown in FIG. 8, the caulked portion 9 is susceptible to damage such as cracks when being formed in order to fixedly fit the inner rings 3a, 3b on the shaft member 2. Further, at the time of formation of the caulked portion 9, force is exerted, in a diametrical outward direction, on the inner peripheral surface of the inner ring 3b adjoining the caulked portion 9. In short, in the case of the conventional construction, for the purpose of forming the caulked portion 9, the cylindrical portion 8 is formed in the inner end face of the shaft member 2 in such a way that the inner and outer peripheral surfaces of the cylindrical portion 8 become concentric with each other in the axial direction of the shaft member. Since the outer and inner peripheral surfaces of the shaft member are formed into mere cylindrical surfaces which are concentric with each other, the cylindrical portion 8 is formed with uniform thickness over its entire length. Such a construction requires great force to produce the caulked portion 9 by caulking the protruding edge of the cylindrical portion 8, rendering the caulking operation laborious. Further, the caulking operation entails exertion of great tensile force on the leading edge of the caulked portion 9, rendering the caulked portion susceptible to damage.
Since great force is exerted on the leading edge of the caulked portion when the caulked portion is formed by caulking, the force exerted on the internal peripheral surface of the inner ring 3b from the caulked portion is also increased correspondingly. Although not to a greater extent, the diameter of the inner ring 3b is slightly changed. If there is an increase in the amount of change in the diameter of the inner ring, there arises a risk of the inner ring 3b being subjected to damage such as cracks, and a risk of a change in the diameter of the inner raceways 11, 11 formed along the outer peripheral surface of the inner ring 3b or of deterioration of geometrical accuracy (e.g., roundness or the degree of accuracy of a cross-sectional profile) of the inner ring. This in turn makes it laborious to maintain at an optimum value the preload imparted to the rolling elements 5, 5 interposed between the inner raceways 11, 11 formed along the outer peripheral surface of the inner ring 3b and the outer raceways 10, 10 facing the inner raceways 11, 11, posing the risk of difficulty of ensuring the durability of the hub unit 1.